Historically, there are two general methods of fermenting malt beverages. For the production of ale-type beers, a top-fermentation process is used which utilizes a species of yeast which tends to rise to the top surface of the fermenting wort. The temperature during top-fermentation is conventionally regulated at about 15.degree.-20.degree. C (58.degree.-68.degree. F) throughout the most active period of fermentation. For the production of lager-type beers, a bottom-fermentation process is used which utilizes a species of yeast that remains more or less uniformly suspended throughout the fermenting wort during active fermentation, by natural agitation created by ascending CO.sub.2 bubbles (25), then settles to a more or less compact layer on the bottom of the fermentation vessel as fermentation reaches completion. The temperature during a bottom-fermentation is conventionally regulated at about 10.degree.-15.degree. C (50.degree.-58.degree. F) during active fermentation (1, 2).
It is well known that if the temperature during bottom-fermentation is increased, the fermentation can be accelerated and the fermentation time substantially shortened. However, it is also known that the beer so obtained has an undesirable winey flavor which is not typical of a lager-type beer (3-5).
Brewing science literature has documented that this winey off-flavor in high-temperature lager fermentations is related to increased amounts of beer volatile compounds, variously known as fusel (or "higher") alcohols and esters (6-15).
It is further known that vigorous exogenous agitation, that is, agitation added over and above that naturally occurring in commercial fermentors as a result of the normal evolution of CO.sub.2 by the fermenting yeast, can substantially reduce the fermentation time of lager beer (4-6, 8, 16). However, it is equally well known that this means of accelerating fermentation has a detrimental effect on the beer flavor which, as in the case with high-temperature accelerated fermentations, is related to an increased level of volatile compounds such as fusel alcohols (4-6, 8, 10, 15-17).
Furthermore, it is well documented that the use of higher temperatures (13, 18, 19) or exogenous agitation (16, 18) during bottom-fermentation results in excessive yeast growth. Thus, the aformentioned techniques, which are generally known to shorten fermentation time, do so with the disadvantages of excessive yeast growth and increased volatiles formation which is deleterious to lager beer flavor.